1. Field of the Invention
The invention relates to a process for producing a highly purified food ingredient from the extract of the Stevia rebaudiana Bertoni plant and its use in various food products and beverages.
2. Description of the Related Art
Nowadays sugar alternatives are receiving increasing attention due to awareness of many diseases in conjunction with consumption of high-sugar foods and beverages. However many artificial sweeteners such as dulcin, sodium cyclamate and saccharin were banned or restricted in some countries due to concerns on their safety. Therefore non-caloric sweeteners of natural origin are becoming increasingly popular. The sweet herb Stevia rebaudiana Bertoni, produces a number of diterpene glycosides which feature high intensity sweetness and sensory properties superior to those of many other high potency sweeteners.
The above-mentioned sweet glycosides, have a common aglycon, steviol, and differ by the number and type of carbohydrate residues at the C13 and C19 positions. The leaves of Stevia are able to accumulate up to 10-20% (on dry weight basis) steviol glycosides. The major glycosides found in Stevia leaves are Rebaudioside A (2-10%), Stevioside (2-10%), and Rebaudioside C (1-2%). Other glycosides such as Rebaudioside B, D, E, and F, Steviolbioside and Rubusoside are found at much lower levels (approx. 0-0.2%).
Two major glycosides—Stevioside and Rebaudioside A, were extensively studied and characterized in terms of their suitability as commercial high intensity sweeteners. Stability studies in carbonated beverages confirmed their heat and pH stability (Chang S. S., Cook, J. M. (1983) Stability studies of stevioside and Rebaudioside A in carbonated beverages. J. Agric. Food Chem. 31: 409-412.)
Steviol glycosides differ from each other not only by molecular structure, but also by their taste properties. Usually stevioside is found to be 110-270 times sweeter than sucrose, Rebaudioside A between 150 and 320 times, and Rebaudioside C between 40-60 times sweeter than sucrose. Dulcoside A is 30 times sweeter than sucrose. Rebaudioside A has the least astringent, the least bitter, and the least persistent aftertaste thus possessing the most favorable sensory attributes in major steviol glycosides (Tanaka O. (1987) Improvement of taste of natural sweetners. Pure Appl. Chem. 69:675-683; Phillips K. C. (1989) Stevia: steps in developing a new sweetener. In: Grenby T. H. ed. Developments in sweeteners, vol. 3. Elsevier Applied Science, London. 1-43.)
Methods for the extraction and purification of sweet glycosides from the Stevia rebaudiana plant using water or organic solvents are described in, for example, U.S. Pat. Nos. 4,361,697; 4,082,858; 4,892,938; 5,972,120; 5,962,678; 7,838,044 and 7,862,845.
However, even in a highly purified state, steviol glycosides still possess undesirable taste attributes such as bitterness, sweet aftertaste, licorice flavor, etc. One of the main obstacles for the successful commercialization of stevia sweeteners are these undesirable taste attributes. It was shown that these flavor notes become more prominent as the concentration of steviol glycosides increases (Prakash I., DuBois G. E., Clos J. F., Wilkens K. L., Fosdick L. E. (2008) Development of rebiana, a natural, non-caloric sweetener. Food Chem. Toxicol., 46, S75-S82.)
Some of these undesirable properties can be reduced or eliminated by subjecting steviol glycosides to the reaction of intermolecular transglycosylation, when new carbohydrate residues are attached to initial molecule at C13 and C19 positions. Depending on the number of carbohydrate residues in these positions the quality and potency of the compounds taste will vary.
Pullulanase, isomaltase (Lobov S. V., Jasai R., Ohtani K., Tanaka O. Yamasaki K. (1991) Enzymatic production of sweet stevioside derivatives: transglycosylation by glucosidases. Agric. Biol. Chem. 55: 2959-2965), β-galactosidase (Kitahata S., Ishikawa S., Miyata T., Tanaka O. (1989) Production of rubusoside derivatives by transglycosylation of various β-galactosidase. Agric. Biol. Chem. 53: 2923-2928), and dextran saccharase (Yamamoto K., Yoshikawa K., Okada S. (1994) Effective production of glucosyl-stevioside by α-1,6-transglucosylation of dextran dextranase. Biosci. Biotech. Biochem. 58: 1657-1661) have been used as transglycosylating enzymes, together with pullulan, maltose, lactose, and partially hydrolyzed starch, respectively, as donors of glycosidic residues.
The transglucosylation of steviol glycosides was also performed by action of cyclodextrin glucanotransferases (CGTase) produced by Bacillus stearothermophilus (U.S. Pat. Nos. 4,219,571, and 7,807,206) as a result α-1,4-glucosyl derivatives were formed with degree of polymerization up to 10.
It was shown that the taste profile and sweetness power of glucosyl derivatives are largely dependent on number of additional glucosyl derivatives, i.e. the degree of polymerization of the α-1,4-glucosyl chain. The increase in number of α-1,4-glucosyl residues improved the taste quality but at the same time reduced the sweetness level. The treatment of transglucosylated stevioside with β-amylase resulted in a product consisting of mono- or di-α-1,4-glucosyl derivatives with better taste profile and optimal sweetness level (Tanaka, 1987). However in such process, the resulting product contains high level of initial unreacted (unmodified) glycosides (generally >20%) which makes it not compliant with regulatory requirements of less than 15% unreacted glycosides (α-Glucosyltransferase Treated Stevia, Japan's Specifications and Standards for Food Additives, VIII edition, 2009, p. 257). Therefore additional steps for chromatographic separation of unreacted steviol glycosides are used to reduce initial unreacted (unmodified) glycosides' content. However chromatographic separation techniques generally involve high cost and are not suitable for large scale production.
It is to be noted also that many glucosyl stevia products contain up to 20% residual dextrins which do not possess significant functional properties and reduce the content of steviol glycosides in the product.
Therefore it is necessary to develop simple process of preparation of high purity glucosyl stevia products with optimal α-1,4-glucosyl chain length and low unreacted glycosides level.